I (and a few other friends) have noticed Padmapper's results seem
less complete than usual lately.

Coincidence?

I would think than an organization like Padmapper would have the
technical and financial wherewithal to build out their own network of
scraper nodes apart from the tor network.

This would be almost impossible to block especially if they stood up
the infrastructure on a large cloud providers where instances could be
re-provisioned with new IP address in numerous cities all over the
globe in a matter of minutes with a click of the button.

I'm not so sure that that would work particularly well, humans rarely
live in datacenters, and it's tough to make cloud IPs look and act the
same as residential IPs, especially when other IPs in the same /24 (or
larger) are owned by different customers. User behaviour would also be
quite different, and it would probably be difficult to mimic typical
human patterns of usage while scraping enough information to be
worthwhile before Craigslist pulls the plug.

Tor exit nodes, on the other hand, have a lot of human shields using
them too, so it makes it a lot harder to narrow down a specific "bad
actor" without also hitting actual users.

So while Tor isn't necessary an ideal choice here, it has some
advantages over dynamically allocating and dropping cloud IPs.

I'm curious why Craigslist doesn't just sell their listing data via
API access to companies like Padmapper, that would be a win-win.

Because they're actively hostile to creating a better user experience.
Don't get me wrong, the fact that their website doesn't look like
someone from marketing took a dump all over it is part of what is
awesome about it, but still...